Manufacture of isoprene



Nov. 19, 1963 J. F. MAGNESS MANUFAGTURE oF ISOPRENE Filed May 9, 1960 United States This invention relates to the manufacture of isoprene and has as a panticular object the provision of an improved, integrated process for the conversion of isopentane to isoprene by catalytic dehydrogenaticn.

Numerous methods are known for isoprene synthesis and 1a variety of processes for the production of isoprene by the catalytic dehydrogenation of isopentane have heretofore been proposed. All such dehydrogenation processes result in the formation of a dehydrogenate which contains, besides unconverted isopentane feedstock, various amounts of C isomcric dehydrogenation products including both monooleiinic and dioleiinic constituents. rhe successful production of isoprene by such processes rests in large part upon the provision of simple, edective and economical means `for separating isoprene from closeboil ing isomers, and upon maximum utilization of partial dehydrogenation products as recycle streams so as to maximize the conversion of isopentane to isoprene. lt has been recognized heretofore that the recycle of pentadienes such as piperylene and cyclopentadiene to the dehydrogenation reactor is undesirable, since these isomers cannot be converted to isoprene and result in increased coke formation in the reactor.

The formation of minor but significant amounts of straight chain C5 diolelins including cis and trans-piperylene and cyclopentadiene during the dehydrogenation of isopentane has made the separation of pure isoprene from the dehydrogenate particularly ditiicuit, because of the close boiling points of these isomers to that of isoprene, and the close chemical similarity of the various pentadienes. Prior art processes for the recovery o-f isoprene from isopentane dehydrogenation have invariabiy provided tirst for the separation of a fraction consisting primarily of diolens, including isoprene, followed by separation of the isoprene contained in said fraction by extraction or distillation means. Typically, the dehydrogenation product is distilled to remove C4 hydrocarbon and lower boiling materials, and the C5 hydrocarbons extractiveiy distilled using aqueous acetone, to remove a distillate rich in paraiins and monoolens. The bottoms from such extractive distillation, comprising C5 diolens including isoprene, other pen-tadienes and 2-methyl-2-butene is then super fractionated or subjected to a second extractive distillation to yield high purity isoprene as an overhead product, and la bottoms stream comprising pentadienes and 2-methyl-2-butene. Alternative processes for removing isoprene from Ithe C5 dehydrogenate are known, for e"- ample by reaction of the dehydrogenate with SO2, to form sulfones from the dioleiins present therein, or by treatment with copper sal-ts such Ias copper ammonium acetate to complex Ithe dioletins with the copper salt, followed by decomposition of the so formed suifones or copper coinplexes to regenerate the dioletins. Each of these processes again depends upon the separation of `a diolen fraction which requires further treatment for separation of isoprene from diolefmic materials isomeric therewith.

I have now discovered that isoprene of extremely high purity can be obtained in sa simple, economic manner from isopentane dehydrogenation products by rst subjecting the C5 raction thereof to super fractionation to separate an isoprene-rich distillate substantially free of other isomeric dioleiins and a bottoms product rich in said isomeric dioletins. The thus obtained isoprene-rich distillate is then subjected to extractive distillation to yield in one step Y a highly purified isoprene concentrate having commercial atent ice utility. My process further contemplates removal of the diolenic constituents from the super-fractionator bottoms prior to recycle thereof to the dehydrogenation reactor, to avoid coke formation and/ or undesirable build up of unwanted isomers such as piperylene in the recycle stream. The process of the present invention is particularly advantageous in that, by rst effecting sepanation of isoprene from other dioleiinic isomers, a process recycle stream 1s provided which can readily be treated for removal of such undesirable dioleiins without loss of isoprene.

in the practice of the present invention, an isopentane feed is contacted at a temperature of from about 1000 F. to about i260 F. with a dehydrogena-tion catalyst such as an `oxide of group VI metal. Commercially available catalysts consisting of about 2%-l0% chromia supported on activated alumina are particularly effective. The dehydrogenaticn is eected :at atmospheric or sub-atmospheric pressure, preferably l-ltl p.s.i.a. Space velocity (liquid volume per hour per volume of catalyst) is advantageously within the range of 0.5 to about 5.

rihe eiiiuent from the dehydrogenation reactor is then treated in conventional stripping and debutanizing apparatus to provide a C5 hydrocarbon stream suitable for further purication to eiect separation of pure isoprene. The C5 hydrocarbon stream is super fractionated to yield a distillate containing isoprene and most of the paraiins and monoolens. The bottoms, comprising primarily straight chain pentadienes, cyclopentadiene tand Z-methyl- 2-butene are then passed to a polymerization zone for conversion `of the dioleiinic constituents thereof Ito dimers and/ or low molecular weight polymers having boiling points substantially higher than those of the monooleiins present in the bottoms stream. Simple yfractionation of the polymerization zone effluent yields a diolen-free stream which is recycled to the dehydrogenation reactor.

isoprene-rich distillate from the super fractionator is processed by eX-tractive distillation utilizing aqueous acetone as solvent for recovery of high purity isoprene as a bottom product, and a distillate which, after washing to remove acetone solvent, is recycled to the dehydrogenation reactor.

In order lthat those skilled in the Iart may fully appreciate the nature of my invention and a method for carrying it out, it will be more specically described in connection with the accompanying drawing, which is a flow sheet of one form of the invention.

in operating in accordance with the process of the present invention, an isopentane feedstock admixed with about 2.3 volumes of recycle hydrocarbon is introduced through line l to furnace 2 in which it is preheated to Iabout 1100 F. The feed is then passed via line 3 to catalytic dehydrogenator i ywherein it is contacted with a catalyst consisting of chromia deposited on activated alumina `at a pressure of 3.5 p.s.i.a. and a space velocity of 3,0 liquid volumes per hour per volume of catalyst. Under these conditions, about 3() percent of the total feed is converted to final products. The distribution of the major products is Wt. percen-t of Product: converted feed lsoprene v59.() Light ends 27.7 4Normal C5 hydrocarbon 3.5 Colze 9.8

The hot product gases pass through line 5 to quench zone 6 which may consist of one or more towers wherein the product gases me cooled by contacting with recycle quench oii introduced via line 7 from absorber-stripper section 1i. lPreferably, two stages a-re employed, the eti'luent product vapor then passing via line 8 to compressor 9 wherein the product vapor is compressed to Ia pressure of about 45 p.s.i.g. and further cooled. The dehydrogenate then passes through line -to a conventional oil absorption and stripping system 11 for recovery of the Cf-andheavier components in the product. Absorber stripper irl serves to remove dehydrogenate from the quench oil employed in zone 6, rich oi-l being introduced to if. via line 12. Uncondensible gases as well as CYS3 hydrocarbons are removed via line 13 and recovered material is passed through line 14 to debutanizer 15 where any remaining butane and lighter material is removed overhead through line 16 for further processing. The C5 and heavier product is then `fed via line l17 to the pu-riiication section. Typically, such C5 fraction may have the following composition:

Weight percent Butylene .0l Butane .01

3-methy-l-1-butene lsopentane l-pentene Z-methyll-butene 8.17 lsoprene 20.2 2-pentene .24 n-Pentane Cyclopentadie-ne .04 24methyl2butene 12.92 Pentadiene 1.14 Absorption oil .95

The iirst step in purification is super `fractionation in column 18 which may be any of the usual `forms of fractionating devices such `as a column containing bubble cap plates. In normal operation of a process according to the invention, between 50 and 60 plates are commonly employed. This column is operated at a pressure of about 20 p.s.i.g. and at a retlux ratio (rellux to distillate product) of about 4/1 to 5/ 1. Under these conditions, the temperature at the top of the tower is usually about 135 F. and an overhead product is obtained from tower 18 containing over 98% of the isoprene contained in the dehydrogenation eilluent, together with unconverted isopentane .and a major proportion of the monoolelinie constituents of the dehydrogenate other than 2-methyl-2- butene. Bottoms from tower 18 contain approximately 90% of the cyclopentadiene and over 99% of the pentadienes other than isoprene originally present in the dehydrogenation eiiluent. The major component of the bottoms is 2-methyl-2-butene. Employing the :feedstock of the specific illustration, the distillate and bottoms from such distillation have the following composition (weight percent):

The distillate stream from tower 12 is passed through line .i9 to an extraction section which consists of an absorber tower 20 and stripper tower 21. ln tower 20, the distillate is contacted with an acetone-water solution (8S-15 weight percent respectively) as the extractant in the ratio of two Volumes of extractant per volume of hydrocarbon liquid iowing down the column. Under these conditions, the less polar materials desired for recycle (isopentane and methylbutenes) or their acetone azeotropes are removed as distillate. After a Water Wash, (not shown) this stream is recycled via line 22 to the reactor. The bottoms product from this extractive distillation is passed through line 23 to stripper column 21, recovering product isoprene as the distillate. Solvent obtained as bottoms from column 21 is recycled via line Zd to absorber 20. The stripper distillate removed through line 2S is [Water washed (not shown) to remove residual acetone solvent, and the product, containing over 97% isoprene `and less than 0.01 part of cyclopentadiene and less than 0.05 part straight chain pent-adienes per 100 parts isoprene, meets specifications for polymerization grade isoprene. ln the processing of the feedstock of the speciiic illustration, the composition of the product isoprene stream obtained under the particular operating conditions given shows the following analysis (weight percent):

Extraetive distillate, isoprene product Returning now to tower 1S, the bottoms therefrom comprising about 75% of lthe 24methyl-2-butene formed in the reactor together with cyclopentadiene and pentadieues is passed via line 26 to selective polymerization zone 27 wherein the bottoms are treated for conversion of cyclopentadiene and pentadienes to higher boiling polymerization products. ln polymerization zone 27 the bottoms product is heated to a temperature between about 200 F. and about 300 F. thus effecting conversion of pentadienes and cyclopentadiene to higher boiling products. Polymerization of the dioleiins in this stream can be effected by contacting with solid phosphoric acid catalyst (phosphoric -acid supported on lkieselguhr) or alternatively with metallic sodium at to 300 F. to effect conversion of dioletinic materials to dimers or higher polymers without ailecting the monoolelinic cornpounds contained therein. I prefer to employ metallic sodium, in particulate form for this purpose, at a temperature of about 200 F. employing between about 0.1 and about 1% sodium by weight based on the dioletlnic content of the bottoms product. Eluent ,from polymerization zone 27 is then passed via line 28 to rerun column 29 wherein the dioleiin-free recycle material, primarily Z-methyl-Z-butene is separated lfrom higher boiling dioleinic conversion products by simple fractionation. Bottoms from column 29 are discarded via line 30, overhead being recycled via line 31 to reaction feed line 1.

The foregoing disclosure and illustration of the nvention demonstrate the remarkable simplicity and eiectiveness by which isoprene of high purity can be obtained by the process of the invention. Various lcombinations and variations of the application of the principles described and illustrated herein will be apparent to those sn'lled in the art and resort may be had to such moditications `and variations as fall within the spirit of the invention and the scope of the appended claims.

l claim:

1. An improved process for the production of isoprene which includes the steps of passing a feedstock comprising isopent-ane to a dehydrogenation zone, contacting it therein with a dehydrogenation catalyst at a temperature of from about l000 F. to about l200 F. recovering Ia reaction product, separating a fraction from said reaction product consisting essentially of live carbon hydrocarbons, subjecting said fraction to fractional distillation to obtain a distillate stream containing isoprene and substantially free of cyclopentadiene and straight chain pentadienes, and a bottoms stream rich in dioleiinic hydrocarbons other than isoprenc, subjecting said distillate stream to extractive distillation in the presence of acetone to elect substantially complete separation of isoprene from other live carbon hydrocarbons in said distillate stream, treating said bottoms stream under polymerization reaction conditions to polymerize diole nic hydrocarbons therein, separating polymer so formed from associated C5 hydrocarbons and recycling all of the live carbon hydrocarbons with the exception of isoprene to the dehydrogenaton zone in admixture with fresh isopentane,

2. The process of claim 1 wherein aqueous acetone containing about 85% acetone and about 15% water by weight is employed in said extractive distillation in an amount of about two volumes per volume of said distillate stream.

3. The process of claim 1 wherein polymerization of diolefin hydrocarbons in said bottoms stream is eiected by thermal treatment at .a temperature between about 200 F. and 300 F.

4. The process of claim 1 4wherein polymerization of diolen hydrocarbons in said bottoms stream is effected by contacting said bottoms stream with particulate sodiu-m in an amount between about 0.1 and about 1.0% based on the weight of diolenic hydrocarbons in said stream at a temperature in the range of 150 to 300 F.

5. A process for the production of isoprene Iwhich comprises passing a feedstock comprising isopentane to a dehydrogenation zone, `contacting said feedstock therein with a dehydrogenation catcmrt lat a temperature in the range of from about 1000" to `about 1200" F., recovering a reaction product, separating a fraction from said reaction product consisting essentially of live-carbon hydrocarbons, subjecting said fraction to 4fractional distillation to obtain a distillate stream comprising isoprene essentially free of other diolenic hydrocarbons and a bottoms stream rich in dioleiinic hydrocarbons other than isoprene and containing about 90% of the cyclopentadiene and over 99% of the pentadienes other than isoprene originally present in said fraction, separating isoprene from said distillate stream by extractive distillation in the presence of acetone, removing dioletinic hydrocarbons from said bottoms stream by treating said stream under polymerization conditions to polymerize said diolens and thereafter separating therefrom the resulting polymer, and recycling all the tive-carbon hydrocarbons with the exception of isoprene and said diolenic hydrocarbons to the dehydrogenation zone in admixture with fresh isopentane.

References Cited in the le of this patent UNITED STATES PATENTS 

1. AN IMPROVED PROCESS FOR THE PRODUCTION OF IOSPRENE WHICH INCLUDES THE STEPS OF PASSING A FEEDSTOCK COMPRISING ISOPENTANE TO A DEHYDROGENATION ZONE, CONTACTING IT THEREIN WITH A DEHYDROGENATION CATLYST AT A TEMPERATURE OF FROM ABOUT 1000*F. TO ABOUT 1200*F. RECOVERING A REACTION PRODUCT, SEPARATING A FRACTION FROM SAID REACTION PRODUCT CONSISTING ESSENTIALLY OF FIVE CARBON HYDROCARBONS, SUBJECTING SAID FRACTION TO FRACTIONAL DISTILLATION TO OBTAIN A DISTILLATE STREAM CONTAINING ISOPRENE AND SUBSTANTIALLY FREE OF CYCLOPENTADIENE AND STRAIGHT CHAIN PENTADIENES, AND A BOTTOMS STREAM RICH IN DIOLEFINIC HYDROCARBONS OTHER THAN ISOPRENE, SUBJECTING SAID DISTILLATE STREAM TO EXTRACTIVE DISTILLATION IN THE PRESENCE OF ACETONE TO EFFECT SUBSTANTIALLY COMPLETE SEPARATION OF ISOPRENE FROM OTHER FIVE CARBON HYDROCRBONS INSAID DISTILLATE STREAM, TREATING SAID BOTTOMS STREAM UNDER POLYMERIZATION REACTION CONDITIONS TO POLYMERIZE DIOLEFINIC HYDROCARBONS THEREIN, SEPARATING POLYMER SO FORMED FROM ASSOCIATED C5 HYDROCARBONS AND RECYCYLING ALL OF THE FIVE CARBON HYDROCARBONS AND RECYCLING ALL OF THE FIVE CARBON HYDROCARBONS WITH THE EXCEPTION OF ISOPRENE TO THE DEHYDROGENATION ZONE INADMIXTURE WITH FRESH ISOPENTANE. 